Some mountain bikes have energy-absorbing suspensions. In the case of the front wheel, the suspension commonly includes an axially or pivotally acting energy-absorbing assembly that enables the front fork to move relative to the headset. Various forms of trailing arm suspensions are used for the rear wheel. In a typical rear suspension, as shown in FIG. 2, a structure 12 resembling the rear triangle of a conventional bike is pivotally attached to a frame portion 14 at a pivot axis 16 and is coupled to a frame portion 18 by an energy-absorbing and damping assembly 20.
FIG. 3 illustrates diagrammatically a characteristic of presently known suspensions, regardless of type, and regardless of whether they are for the front or rear wheels. The geometry of typical suspensions is such that the displacement of the wheel (and its rotation axis) when the wheel goes over a bump is primarily vertical. This means that the speed at which energy is absorbed by the suspension is short. For example, if the suspension could be designed to permit an infinite displacement horizontally upon encountering a bump, the time for absorbing the displacement would also be infinite--the wheel would never go over the bump. Increasing the time for energy absorption permits the capacity of the energy absorbing and damping components to be reduced, as a direct function of the time, and improves the efficiency of the suspension. The geometries of previously known bicycle suspensions is such that only small horizontal rearward displacements for any given vertical displacements can occur. Some suspensions produce forward horizontal displacements upon upward vertical displacements, which makes the time for response even shorter. Thus, the capacity of the energy-absorbing and damping components of previously known bicycle suspensions must be large to absorb and damp the energy rapidly.
As shown schematically in FIG. 3, a suspension of the type shown in FIG. 2 permits the rear wheel 22 to be displaced upwardly through a distance .DELTA.V when the bike traverses a bump. In most, if not all, rear suspensions of the type under consideration, the pulling distance of the chain 24 between the pedal crank sprocket 26 and the freewheel sprocket 28 changes from a length L1 to a length L2 (which may be either an increase or decrease, depending on the geometry) when the rotation axis 30 of the wheel displaces vertically. Thus, the suspension is coupled to the chain pull, in the sense that displacement of the suspension either drives or releases the driving sprocket wheel due to the change in pulling length. When the pulling length L2 is increased over L1 (pulling length of chain increases), the rider experiences an abrupt decrease of the pressure from the pedals to his legs as the increase in length is taken up by rotation of the driving sprocket wheel relative to the driven sprocket wheel, followed by a resumption of full pressure. When the pulling length of the chain decreases, the rider experiences an abrupt increase in the pressure from the pedals to his legs as the driven sprocket tends to drive the driving sprocket in reverse to accommodate the reduced pulling length, followed by a resumption of normal pressure. In either case, the changing forces on the legs as the pulling length changes is bothersome. Another way of looking at the way the suspension works is to recognize that the forces of the rider's legs on the driving sprocket wheel work in conjunction with the energy-absorbing and damping system of the suspension in responding to traversing bumps. Thus, the rider is functioning as a shock absorber.